Knocking noise when clutch pedal is disengaged under load

[Tataocb]

I have an 88 C1500 5.7 TBI with a manual transmission. The odometer reads 260k miles. I bought it a few months ago and have not driven it much. The previos owner said the engine and transmission were rebuilt by his father in law, so he had no paperwork, which makes me doubt that is true.
I don't know if this is normal, but while slowly going up my inclined driveway (not super steep), with the clutch pedal almost all the way out and the car barely moving, I could hear a knock. As soon as I pressed the pedal all the way on, the knock goes away. If I give I pump the accelerator with the clutch pedal all the way in, then I don't hear anything. But if I let the clutch pedal out to that spot where it is about to start moving or shut off if I don't give it gas, the. I can hear that knock again.
Is this normal or the early stages of the engine developing a knock? The oil was changed a couple weeks ago, probably haven't driven it more than 10 miles since the oil change.
Here is a video I recorded. When I say the clutch is almost all the way out, it is at that spot where the truck wants to start moving, but I have blocks in front and behind the tires.

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[turboedge]

Is that just the sound of the clutch grabbing a little and causing gears to make noise?

 

[L31MaxExpress]

Could be the thrust bearing surface starting to go. See if you can make the crank walk front and rear.

 

[RichLo]

Maybe a loose motor or trans mount hitting something during that specific set of events?

 

[Airmanwoody]

Could also be the throw out bearing in the clutch. I've had a few of those fail and they can act as you describe when they start to go.

I doubt your engine has developed an intermittent knock dependent on load and clutch inputs.

 

[DeCaff2007]

Could also be the throw out bearing in the clutch. I've had a few of those fail and they can act as you describe when they start to go.

I doubt your engine has developed an intermittent knock dependent on load and clutch inputs.

I can't see the vid at work, but this is the cause that I'd go with.

 

[Tataocb]

Thanks for the feedback. I have never heard of an intermittent knock like Airmanwoody mentioned either, but was not sure if it might be some sort of really faint knock that intensifies with the clutch position. All the explanations provided make sense. The engine runs well, but at 260k miles, I fear it can go out at anytime lol.

 

[Airmanwoody]

Thanks for the feedback. I have never heard of an intermittent knock like Airmanwoody mentioned either, but was not sure if it might be some sort of really faint knock that intensifies with the clutch position. All the explanations provided make sense. The engine runs well, but at 260k miles, I fear it can go out at anytime lol.

I still drive my dad's 02 Silverado with 300k on the original engine and trans everywhere it's needed. I even pull the boat and snowmobiles with it. As long as I don't overload it I have no worries using it. I know we maintain it properly and repair it when necessary. I'm sure it won't last forever, but I've had newer vehicles fail and it's far more frustrating to have that happen, versus the older one that I have gotten my money's worth out of. I carry towing coverage on all my vehicles just in case I need it.

Also, is your oil clean? Or does it look like you have metal in it? If you have a rod knock, you'll find metal in the oil.

 

[Tataocb]

I still drive my dad's 02 Silverado with 300k on the original engine and trans everywhere it's needed. I even pull the boat and snowmobiles with it. As long as I don't overload it I have no worries using it. I know we maintain it properly and repair it when necessary. I'm sure it won't last forever, but I've had newer vehicles fail and it's far more frustrating to have that happen, versus the older one that I have gotten my money's worth out of. I carry towing coverage on all my vehicles just in case I need it.

Also, is your oil clean? Or does it look like you have metal in it? If you have a rod knock, you'll find metal in the oil.

The oil looked clean when I changed it a couple weeks ago.

 

[Airmanwoody]

Then I'd bet it's your throw out bearing in the clutch.

I've known many people who thought they had an engine dying with a knock only to find out it was a clutch.